Electrowinning with an anode having a multicomponent coating

ABSTRACT

Described is a process for winning metals from aqueous acidic solutions by passing an electrolyzing current between a cathode and an anode, which anode comprises a conductive substrate bearing on the surface thereof a coating of the mixed oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum.

United States Patent Schenker et al.

[451 Feb. 11, 1975 ELECTROWINNING WITH AN ANODE HAVING A MULTICOMPONENT COATING Inventors: Barry A. Schenker, Mayfield Heights; James M. Kolb, Mentor; Charles R. Franks, North Madison, all of Ohio Diamond Shamrock Corporation, Cleveland, Ohio Filed: Apr. 19, 1973 Appl. No.: 352,417

Assignee:

US. Cl 204/105 R, 204/108, 204/129, 204/290 F Int. Cl C22d 1/00, BOlk 3/06 Field of Search 204/290 F, 290 R, 105 R 204/106, 108

[5 6] References Cited UNITED STATES PATENTS 3,701,724 10/1972 Entwisle et a1. 204/290 F 3,775,284 11/1973 Bennett et a1. 204/290 F Primary E.raminer--John H. Mack Assistant ExaminerAaron Weisstuch Allorney, Agent, or Firm--Timothy E. Tinkler [57] ABSTRACT 6 Claims, No Drawings ELECTROWINNING WITH AN ANOI )E HAVING A MULTICOMPONENT COATING BACKGROUND OF THE INVENTION A variety of metals are produced by electrolyzing acidic aqueous solutions containing said metal, i.e., by electrowinning. In such processes, ores, ore concentrates, and other materials containing compounds of the desired metals are leached with acids, usually sulfuric acid, and the leach liquor, after upgrading if necessary, is then electrolyzed between an insoluble anode and a cathode, the latter often being of the same metal as that to be deposited.

In such processes, large amounts of oxygen are evolved at the anode and contribute to the chemical and mechanical attrition of same. Thus, anodes must be resistant in this respect and must also have a low oxygen overvoltage to ensure economical operation from a standpoint of electrical consumption. Perhaps most typically used for this purpose are lead and lead alloy anodes. Lead anodes nevertheless still have a definite wear-rate, and hence a limited life, and lead to contamination of the cathode deposit with small amounts of lead. Thus, the search had continued for anode materials having a suitable chemical and mechanical resistance coupled with the requisite low oxygen overvoltage.

STATEMENT OF THE INVENTION Therefore, it is an object of the present invention to provide a process for electrowinning metals from aqueous solution, which process may be operated for extended periods of time without serious anode deterioration.

It is a further object of the present invention to provide a process for electrowinning metals from aqueous solutions without substantial contamination of the cathode deposit by materials dissolved from the anode.

These and further objects of the present invention will become apparent to those skilled in the art from the specification and claims that follow.

In a process for the electrowinning of metals from aqueous acidic solutions thereof, there has now been found the improvement which consists essentially of passing an electrolyzing current through said solution between a cathode and an opposed anode, which anode comprises an electrically conductive substrate bearing on at least a portion of the surface thereof a mixed oxide coating of from 30 to 90 percent stannic oxide, l'.0 to l percent antimony oxide, 1.0 to 50 percent of at least one platinum group metal oxide, and 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15. Such a process has the advantagcs ol' anodic oxygen evolution at a low oxygen overvoltage, thereby contributing to electrical efficiency; long operation without anode passivation; and substantially no contamination of the cathode deposit by metals dissolved from the anode. By anode passivation, it is intended to refer to an increase in operating voltage to a value at which operation no longer may be considered practical (e.g., 8 volts) brought about to a substantial extent by the evolution of oxgen at an anodic surface with consequent formation of nonconductive oxides.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Basically, the invention encompasses the deposition of metals from aqueous acidic solutions by passing an electrolyzing current through said solution between at least one cathode and a specific, opposed anode within an electrolytic cell. Generally, the: cell itself need comprise little more than an open box, usually rectangular. in shape, with the appropriate inlets, outlets, and electrical current conductors and distributors. Such cells may be constructed of any suitably resistant material. for example, PVC-lined concrete. The cathode may be of the same metal as that to be won or may be different and may have any configuration suitable to the process and cell design, e.g., a metallic sheet provided with electrically conductive means (hooks) to depend same from a current collecting bar traversing the cell.

A variety of metals may be won from solution according to the practice of the present invention, including cobalt, zinc, iron, manganese, chromium, nickel, copper, and cadmium. While the specific details of operation depend upon the metal to be deposited, as is de' scribed more fully hereinbelow, generally operation is at a pH of less than 7, and preferably less than 4. This acidity is contributed by the acid used to leach the metals into solution from ores or other materials containing compounds of same, which acid is usually regenerated in the electrolytic process. Obviously, a large excess of acid is uneconomical and corrosive.

The temperature range of the process may be between room temperature and the boiling point of the solution in question. While higher temperatures lower the electrical resistivity of the solution, operation at elevated temperatures contributes to a lower current efficiency, excessive evaporation, and exaggerates the effect of impurities in the solution upon the metal being deposited. Therefore, it is necessary to strike a balance between these considerations and operate somewhat above room temperature but considerably below boil' ing, this often requiring the provision of cooling means.

The invention contemplates the use of any of the additive materials conventional to electrowinning appli cations and useful in reducing'the roughness of the deposit with increasing thickness, in reducing the effect ofimpurities on the electrical efficiency, and/or in providing a demisting foam blanket on the surface of the electrolyte.

Thus, the solutions to be treated are prepared by leaching ores, ore concentrates, metallurgical byproducts, and the like, with an aqueous acid, usually sulfuric, followed in many instances by purification (e.g., cementation), especially if the undesired metals in solution are more noble than the metal to be won. The solution is then ready for electrolysis using the anodes according to the present invention.

A critical feature of the present invention is the use of a particular anode which, as mentioned above, is a conductive substrate bearing on at least a portion of the surface thereof a coating of mixed oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum. With the use of such anodes, the tendency towards passivation when oxygen is evolved at the anodic surface is greatly reduced and hence the anodes exhibit a long useful lifetime. Since said anodes are substantially completely mechanically inert in the cell environment, no

materials capable of contaminating the cathode deposit are produced.

The conductive substrate is preferably titanium, although tantalum, niobium, and zirconium may also be employed. ln addition, a covering of one of the aforementioned metals over a more conductive material, such as copper or aluminum, may be used. Further, layers on the substrate intermediate the base metal and the coating, such as those described in U.S. Pat. No. 3,71 1,397, are contemplated. The configuration of the substrate may vary considerablybut is generally in the form of rods or an imperforate or foraminous metal sheet, e.g., of titanium metal.

What may be considered the first of the components in the coating composition is stannic oxide, preferably present in the form of crystalline SnO and employed within the range of from 30 to 90, especially 30 to 50, percent by weight of the total coating composition.

The antimony oxide component enters into the tin oxide crystal lattice, rendering same more electrically conductive. Although the antimony is present in an indeterminate oxide form owing to its entrance into the stannic oxide crystal lattice, it is expressed for convenience sake as Sb O Thus, on this basis, the antimony oxide is present within the range from 1.0 to 10, preferably 4.0 to 8.0, percent by weight.

The foregoing ranges of tin and antimony oxides are further qualified by the proviso that they be present, respectively, in the range, on a mole ratio basis, of 95:5 to 85:15, especially 90:10. In this fashion, there is obtained the desired doping effect of the antimony on the tin oxide without the presence of an excess separate phase of antimony oxide.

The third component of the mixed coating is at least one platinum group metal oxide," by which term it is intended to include the oxides of platinum, palladium, ruthenium, iridium, rhodium, and osmium, especially those of ruthenium and iridium. These platinum group metal oxides are present for the most part in their most highly oxidized state and within the range of from L to 50, especially 20 to 40, percent by weight. An especially preferred anode is one the coating of which contains a combination of Ru0 and [r0 or rhodium oxide.

The final component is a valve metal oxide selected from the group consisting oftitanium and tantalum oxides. Whereas the titanium is present in the form of TiO and is essentially crystalline (rutile) in nature, when tantalum is employed, as in the preferred embodiment, a generally amorphous tantalum oxide results. Therefore, although it is expressed as Ta O it is understood that mixtures of tantalum oxides may in fact be present. The amounts of valve metal oxide employed are generally within the range of from 0.5 to 30 percent by weight, especially to 25.

Thus, a preferred electrode comprises a titanium substrate bearing a coating containing about 47 percent SnO 5 percent Sb O 23.5 percent RuO 4.5 percent ho and percent Ta O While many of the variety of methods known for producing mixed metal oxide coatings may be employed, the preferred method of preparing the multicomponent coating composition on the titanium substrate is by deposition from a solution of the appropriate thermochemically decomposable salts. For example, it is desirable to paint or brush an acidified alcoholic solution of said salts onto the substrate followed by drying at l00-l40C for from 3 to 10, especially 5, minutes and finally by baking in an oxidizing atmosphere, e.g., air, at 450 to 520C, especially 500C, for from 5 to 10, especially about 7, minutes. This procedure may then be repeated any number of times until the desired coating thickness is obtained, for example, 6 to 10 coats. The preferred solvents for the thermally decomposable salts are the lower alkanols such as ethanol, propanol, amyl alcohol, and especially n-butyl alcohol, although other solvents including water may be employed. To these solvents there is generally added from 0 to 50 percent by volume of an acid, such as hydrochloric acid. The concentration of the metals in the solution from which the coating composition is to be derived ranges between about 50 to 200 grams per liter. The salts employed are generally any thermally decomposable inorganic or organic salt or organic ester of the metals in question such as the chlorides, nitrates, alkoxides, alkoxy halides, resinates, amines, and the like. Specific and illustrative examples include potassium hexachloroplatinate, hexachloroiridic acid, ruthenium trichloride or tribromide, orthobutyl titanate, antimony trichloride or pentachloride, and stannic chloride or dibutyl tin dichloride.

It will be understood by those skilled in the art that it is possible to use a number of combinations of preformed oxides of the various component metal and salts of the remaining materials, although it is generally believed that preformed valve metal oxides should not be employed nor should separately preformed'tin and antimony oxides be used. Further, if thermal decomposition is incomplete, small amounts of salts may remain without detrimental effect in the coating, for example. small amounts of chloride in the primarily oxide coat- Perhaps most exemplary of the process of the present invention is the electrowinning of copper from an aqueous solution comprising copper sulfate and sulfuric acid employing a copper sheet cathode and anode according to the present invention. During electrolysis, generally occuring at a current density within the range of 15 to amperes per square foot, sulfuric acid is regenerated and copper is deposited at the cathode. The pH of the electrolyte will range from less than 1 up to about 2.0 and a temperature of 30 to 65C is most commonly employed. In order to obtain a chemically pure, physically dense, and adherent cathode deposit, the concentration of the electrolyte must be controlled, as severe depletion cannot be allowed. A typically advantageous amount of copper in the solution is within the range of 25 to 50 grams per liter. Of course, the presence or absence of other metals will depend upon the nature of the material leached.

In a process wherein zinc is the element to be won, electrolysis is generally of a zinc sulfate solution that has usually been purified to some extent by a cementation reaction. Employing, for example, an aluminum sheet cathode and a current density within the range of 30 to 100 asf, electrolysis is of a solution containing from 40 to 50 gpl zinc at a pH of from less than 1.0 to about 2.0 and a temperature within the range of 30 to 40C.

With cadmium, aluminum cathodes may again be employed in the electrolysis of a solution containing from 100 to 200 gpl cadmium at a pH of from less than 1 to about 1.5, a temperature within the range of 20 to 35C and at a current density of 4 to 10 asf.

The electrowinning of chromium from solution requires the additional presence of a diaphragm to prevent mixture of the chromic and sulfuric acids formed at the anode with the catholyte and hence undesirable EXAMPLE 2 An anode coating solution is prepared from 45 ml ethanol, 4.5 g TaCl 1.1 g SbCl 15.1 g SnCl .5H O, and 7.6 g RuCl .xH- O (38Ru). An etched titanium l f i chrome; 3: fgb g 5 mesh substrate is coated by brushing, drying at 1 10, C zzg z hggf xg i 5 e Ca for 3 minutes, and baking at 500 C for 7 minutes. The y p n um epo coating procedure is repeated until a coating having a ally on a stainless steel cathode at a current denslty ruthemum content of 1 gram per square foot is obwithin the range of 45 to 70 asf and a temperature on l o O tamed. This is labeled Anode 4. theorderof50 to60C. 10 A d f Electrowinning of manganese likewise takes lace in .110 e prepare m an entlca] ashlon but Submtutmg 0.92 g of lrCl and 6.54 g RuCl .xl-l O for the rua diaphragm type cell employing a feed contalnmg bethemum content of Anode 4. Anode 6 is likewise simitween and gpl manganese as manganese sulfate lar with th ex th I l 28 f Rhcl 3H 0 d together with ammonium sulfate. Reduction of manga- 6 65 R ij h g 0 an nese occurs on the stainless steel or titanium cathode 15 t g u M 2 comprise t e precious meta at a current density from 15 to 100 asf and a temperac l d l d I d I l lure between 30 and 40C. Sulfuric acid is again regenen eye as e .ectiowmnmg es i evd ermed in the anolyte compartment. ated accordmg to the lifetime test described 1n Exam- Further processes include the electrowinning of cok 1 above Anodes and 6 have f l f respeci bah (15 50 gpl CO PH l.5 7.0 5 o o and 3O 4O t1vely,of 185, 250, and 350 hours. Th1s indlcates the asf) and nickel (50:10 gpl, pH 304.5 0 0 and 1mproved operation possible employing a mixture of 1540 as precious metal oxides in the coating.

In order that those skilled in the art may more readily It be emphaslzed that the mists applied m the understand the present invention, the following specific foregomgFxamples are acclferated m that hlgher than examples are afforded in which the longevity of operacommerclal clfrrent densmes were employed m tion in a sulfuric acid solution is shown as proof of the achieve results i reasonable length of i f general applicability of the process of the present ma] currentdensltles, on the order of 0.l4 em (.0 asf). vention to electrowinning a variety of metals from the anode l measures m years acidic aqueous solution. we

a 1. In a process for the electrowmmng of metals from EXAMPLE 1 aqueous acidic solutions thereof, the improvement that consists essentially of passing an electrolyzing current A series of electrodes is prepared and evaluated as between a Cathode and an PP yg n ol ing 21nelectrowinning anodes as follows. In each instance, the Said anode mpri ing n electrically Conductive quantity of thermally decomposable salt set forth in the supporting substrate bearing on at least a portion of the table is dissolved in 45 ml of ethanol with stirring. The surface thereof a coating consisting essentially of from resultant solution is brushed onto an expanded titanium I 10 Percent n im ny oxide, as z al On Weight mesh substrate previously cleaned by etching for 30 basis, from 30 t0 90 Percent 2 from to 50 P minutes in boiling (l8%) aqueous hydrochloric acid. c of at least 0118 Platinum g p metal OXide, 21nd The solution is applied to the mesh by brushing, folfrom 15 to 25 percent of a valve metal oxide selected lowed by drying the anode for 3 minutes at l 10C and from the group consisting of titanium and tantalum oxfiring in air at 500C for 7 minutes. This brushing, dryides, with the proviso that the mole ratio of tin to antiing. and baking procedure is repeated until a coating mony oxides is between 95:5 and 85:15. containing 1.7 grams of ruthenium per square foot of 2. A process as in claim 1 wherein the metal to be anode surface is obtained (usually 6-10 coats). Follow- Won lS Selected from the group Consisting of QPP ing the final baking, the electrodes are employed as anzinc, cobalt, iron, nickel, manganese, cadmium, and odes in a 150 g/l sulfuric acid solution at 3 amperes per chromium. square inch opposite a titanium mesh cathode and at an 3. A process as in claim 1 wherein the anode coating electrode gap of 2 inches. The test is continued until consists essentially of from 4.0 to 8.0 percent Sbg03, the anodes have passivated, i.e., a voltage of 8.0 volts from 30 to 50 percent SnO from 20 to 40 percent of or greater is obtained. The lifetime of the anode, that at least one platinum metal oxide, and from 15 to 25 the umbe Of hours of Successful operation until percent ofa valve metal oxide selected from the group passivation occurs, is reported in the following table. consisting of titanium and tantalum oxides.

TABLE RuCl .Xl-l O SnCl .5H O SnO SbCl Sb O (38%) RuO TaCl Ta O Lifetime Anode g g g g hrs.

3 l. i 47.2 i l 5.1 7.6 27. 4.5 20.1 650 From the foregoing it is apparent that electrowinning 4. A process for winning copper from an acidic aque processes wherein an acidic aqueous electrolyte is to be ous copper sulfate solution, which process comprises treated can be operated for extended periods of time employing anodes within the definition of the present invention.

passing an electrolyzing current through said solution between a copper sheet cathode and an opposed oxygen evolving anode, which anode comprises an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 1.0 to 10 percent antimony oxide, as Sb O on a weight basis, from 30 to 90 percent SnO from 1.0 to 50 percent of at least one platinum group metal oxide, and from to 25 percent ofa valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15.

5. A process as in claim 4 wherein the copper content LII consisting of titanium and tantalum oxides. 

1. IN A PROCESS FOR THE ELECTROWINNING OF METALS FROM AQUEOUS ACIDIC SOLUTIONS THEREOF, THE IMPROVEMENT THAT CONSISTS ESSENTIALLY OF PASSING AN ELECTROLYZING CURRENT BETWEEN A CATHODE AND AN OPPOSED OXYGEN EVOLVING ANODE, SAID ANODE COMPRISING AN ELECTRICALLY CONDUCTIVE SUPPORTING SUBSTRATE BEARING ON AT LEAST A PORTION OF THE SURFACE THEREOF A COATING CONSISTING ESSENTIALLY OF FROM 1.0 TO 10 PERCENT ANTIMONY OXIDE, AS SB2O3, ON A WEIGHT BASIS, FROM 30 TO 90 PERCENT SNO2, FROM 1.0 TO 50 PERCENT OF AT LEAST ONE PLATINUM GROUP METAL OXIDE, AND FROM 15 TO 25 PERCENT OF A VALVE METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND TANTALUM OXIDES, WITH THE PROVISO THAT THE MOLE RATIO OF TIN TO ANTIMONY OXIDES IS BETWEEN 95:5 AND 85:15.
 2. A process as in claim 1 wherein the metal to be won is selected from the group consisting of copper, zinc, cobalt, iron, nickel, manganese, cadmium, and chromium.
 3. A process as in claim 1 wherein the anode coating consists essentially of from 4.0 to 8.0 percent Sb2O3, from 30 to 50 percent SnO2, from 20 to 40 percent of at least one platinum metal oxide, and from 15 to 25 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides.
 4. A process for winning copper from an acidic aqueous copper sulfate solution, which process comprises passing an electrolyzing current through said solution between a copper sheet cathode and an opposed oxygen evolving anode, which anode comprises an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 1.0 to 10 percent antimony oxide, as Sb2O3, on a weight basis, from 30 to 90 percent SnO2, from 1.0 to 50 percent of at least one platinum group metal oxide, and from 15 to 25 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:
 15. 5. A process as in claim 4 wherein the copper content of the solution is between 25 and 50 gpl, the pH of the solution is less than 2.0, the temperature of operation is between 30* and 65* C, and the electrolyzing current is within the range 15 to 100 asf.
 6. A process as in claim 4 wherein the anode coating consists essentially of from 4.0 to 8.0 percent Sb2O3, from 30 to 50 percent SnO2, from 20 to 40 percent of at least one platinum metal oxide, and from 15 to 25 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides. 